There are two specific test cases:

• jstests/core/map_reduce_validation.js will skip validating that mapReduce fails when run against a view if the fixture is via mongos. After some local testing I believe it should "just work" and we can remove the check that prevents the validation.
• jstests/aggregation/sources/group/group_large_documents.js is marked assumes_against_mongod_not_mongos since a pipeline that is split into a shards and merging part will hit the bson-too-large error when merging. However, it would be possible to test this against a mongos if it is tested against a one-shard cluster as there would be no merging part. The goal would be to explicitly test this scenario.